In bicycles, slowing down or braking a wheel takes place in various ways, including: through the clamping of jaws carrying pads about the rim, in particular on opposite side walls thereof, or more generally about a peripheral portion of the wheel (rim brake); or through pressing pads against a disc fixedly connected to the hub of the wheel (disc brake).
In the present description and in the attached claims, under “braking body”, a pad is meant to be indicated.
In the present description and in the attached claims, under “wheel component”, a rim of a spoked wheel, a rim or peripheral portion of a disc wheel or of a spider wheel, or a disc of a disc brake is meant to be indicated.
More specifically and as well known, a rim of a spoked wheel comprises an annular body intended to be coupled, on a radially outer side, with a tire of the bicycle wheel and, on a radially inner side, with a plurality of spokes of the bicycle wheel, that are in turn coupled with a hub of the wheel. Typically, the annular body of the rim comprises opposite side walls. On the side walls there are brake tracks or areas, on which the pads of a brake of the bicycle act.
In the present disclosure and in the attached claims, under “composite material”, a material comprising structural fibers incorporated in a polymeric material, sometimes called “matrix” or “resin” is meant to be indicated.
The polymeric material can be thermoplastic material or thermosetting material.
The structural fibers typically comprise carbon fiber, glass fiber, aramid fiber, ceramic fiber, boron fiber and/or combinations thereof. The geometry of the reinforcing fibers is extremely varied, ranging from very short fibers randomly arranged in the matrix, to more or less long fibers arranged in a more or less orderly manner, for example in parallel bundles, in a woven or non-woven bidirectional manner, and/or combinations thereof. Components made of composite material are typically manufactured through compression molding in the case of thermosetting polymeric material, and through injection molding in the case of thermoplastic polymeric material. The starting material can also take up various configurations, for example it can comprise dry fiber or fiber preimpregnated with polymeric material; the preimpregnated fiber can be in the form of “mats”, of loose conglomerates, or even of “mats” of conglomerates, wherein under “conglomerates”, substantially two-dimensional small pieces or three-dimensional small pieces of fiber segments are meant to be indicated, inside which conglomerates the fiber segments are usually arranged parallel to one another or even according to a woven structure; under “mat”, a starting material having a negligible thickness with respect to the width and length dimensions is meant.
Bicycle wheel components made of composite material offer the advantage of combining high rigidity and mechanical strength with a low weight.
According to EP 2 684 707 A1, carbon surfaces—such as those of a carbon fiber rim exposed after a short period of brake use when a mold release material and the outer layer of epoxy wore away—are relatively poor at resisting wear and enhancing frictional performance, and thus carbon fiber surfaces are generally unsuitable as brake tracks for fiber reinforced plastic (FRP) rims. It therefore discloses a rim for a bicycle, including a radially outer tire-engaging portion, a radially inner spoke-engaging portion, a first sidewall, a second sidewall spaced apart from the first sidewall, the first and second sidewalls extending inwardly of the radially outer tire-engaging portion, and a brake track disposed on the first and second sidewalls comprising a layer of non-fibrous microparticles such as high hardness, high thermal conductivity and high thermal resistance microspheres of silicon carbide, silicon nitride, boron carbide, ceramic, metallic particles i.a. The microparticles are embedded, preferably partially embedded in an epoxy matrix and partially exposed. The range of microparticles to epoxy resin is preferably about 5-60% abrasive microparticles, by weight. The method of manufacturing involves the application to an FRP rim of a track of non-fibrous microparticles of silicon carbide or the like, suspended fully or partially in a layer of high temperature, high toughness epoxy resin; then the removal of an amount of epoxy from the brake track to fine tune the coefficient of friction of the brake track as well as the surface topography. In this manner, the FRP can be optimized for wet-weather braking. More specifically, according to the document, after curing the rim, excess resin is removed and the rim is ready to have the outmost part of the epoxy of the brake track removed to expose the microparticles. According to the document, this step may not be necessary but wet weather braking performance on a new carbon fiber rim is typically poor and the surface may take weeks to “break in” as brake pads slowly abrade away the epoxy to expose the microparticles embedded within. Therefore, the epoxy covering the microparticles is lightly abraded away in an abrasive blasting operation. This blasting can be fine-tuned using a media such as garnet which is harder than the epoxy resin but less hard than the abrasive particulates molded into the brake surface.
The Applicant observes that the addition of the microparticles causes undesirable processing costs and time, in addition to being a processing that is not very repeatable.
Document EP 1 625 028 B2 discloses a rim for bicycles and the like, comprising at least one braking area on at least one flank of the rim for placing a braking member, said braking area essentially consisting of fiber-reinforced plastic in the form of a layered semi-finished reinforcing fiber sheet product, wherein the surface of the braking area exhibits a percentage of exposed reinforcing fiber of more than 10% and up to 90%, the reinforcing fiber being exposed by removal of the material of the braking area.
According to such a document, as a consequence of the compression molding, the excess polymeric material that has been pushed to the surface of the braking area would be a drawback in terms of wear with respect to steel or aluminum rims. In order to avoid this without applying aluminum layers or parts, and without resorting to galvanic coating, and without applying additional hard material, the document teaches to carry out chip removal machining, namely with a cutting tool, or machining by erosion along the entire brake track, during which machining the surface polymeric material is removed and sections of fiber are exposed. According to the document, the rim obtains definite properties that are mainly determined by the physical properties of the fibers, such as good braking behavior, low wear (the wearing of the surface is extremely low and is limited mainly to the rubber brake lining), and good thermoconductive properties (good removal of the heat due to the action of the brakes). In addition, it is possible to provide for grooves to improve wet behavior. The document envisages the use of composite materials different from layered material.
Document EP 2 765 009 A1 recognizes that the machining with a cutting tool of such a solution causes damage to the reinforcing fibers and consequent weakening of the rim, and observes that such a solution in fact makes it necessary to use a material richer in fiber that is subsequently removed at the surface, with a consequent waste of material and increased weight of the rim. In order to overcome said drawbacks by providing a rim with at least one braking area substantially made of composite material that is light, strong and easy to manufacture, document EP 2 765 009 A1 teaches to machine—in particular by laser cutting with a numerical control machine CNC and more in particular through a FIR laser, but more in general through milling, turning, filing or scraping—at least one part of the braking area to form a macroscopic surface structure in order to improve the braking properties, in particular in the wet.
The macroscopic structure is in particular intended to interrupt or evacuate a film of water in the braking area, and in particular comprises narrow cavities extending at least in part according to a radial direction and/or that open towards the outside of the rim. The cavities are preferably made in the resin only, so as not to deteriorate the mechanical properties of the fibers, but vice-versa they can be made in the fiber only or in both of the components of the composite material. The cavities can have a rectangular, triangular or rounded section and others; alternatively, the macroscopic structure can comprise substantially rectangular areas, sectors or the entire annular braking area. The document also teaches to use an irregular macroscopic structure, for example aperiodic and/or different macroscopic structures on the two flanks of the rim to avoid phenomena of resonance. It is provided for the rim to have, on one or both flanks, one or more braking areas, and also for the macroscopic structure to be made only on one braking area or on a part thereof. Moreover, the braking area can be made on a disc of a disc wheel or on a disc of a brake.
In such a document, laser machining is indicated as advantageous since it allows, amongst other things, the selective removal of material of the composite material, in particular the resin, to be controlled in order to obtain a better friction coefficient, in particular for braking in the rain; moreover, it allows cavities to be made having a larger surface without weakening the structure of the wheel. By removing only the surface resin with the laser, the appearance of the carbon fiber fabric remains visible and, according to the document, the surface is neither smooth as it was originally, nor rough like through machining with a cutting tool, rather it is gentle, as a fabric could be; the structure is recognizable to the eye and to the touch, since the removal of the resin produces areas in slight depression and slightly raised areas. The document teaches to select a laser for which the resin is not transparent, preferably a CO2 laser in the far infrared, for example having an emission wavelength of about 10.6 micron, and teaches that the difference existing between the combustion temperature of the resin (between 400° C. and 600° C.) and that of carbon fiber (over 1500° C.) allows the laser to be adjusted so that the carbon fibers are not influenced by the laser. Moreover, the document recognizes that in reality it is not possible to precisely determine the surface thickness of the resin, and thus teaches to adjust the laser so that at least 80% of the resin is removed, and so that at most 20% of the carbon fiber is damaged or cut.
The Applicant observes that the machining of such a document, when carried out in sufficiently large regions, causes a surface that can generate noise during braking; vice-versa when carried out through discrete cavities it does not seem to result in good dry-weather braking performance.